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Topic: engine balance (Read 430 times)

Having just posted an update on the frame registration concerns I had, I was reminded about the engine I will be using and how the engine balance method I used could help those with A10 vibration issues. We hear about 54% and 58% balance factors, but every alteration requires a complete engine strip. Well, not necessarily. Here’s the trick: remove sump plate. Turn engine until pistons are at bottom. Now the flywheel bolt should be in the centre of the crankcase hole. Mark a spot about one inch infront of that bolt and another one inch behind. Mark two more, about one inch infront and behind the first two marks. Now drill those marks 9mm dia to a depth of 12mm. Now open those holes to 9.9mm (10mm will do) and tap to 7/16 UNF. Now make up plugs from 7/16 HT bolts to fit those holes, such that they do not protrude (cut a decent screwdriver slot across them before fitting). Now you have balance weights you can fit / remove until you reach the best balance compromise to suit the rev range you prefer – all without stripping the engine. Below is an example from the crankshaft I made for my vee twin project. Crank is nitrided EN40B and the flywheels – drilled and tapped for balance weights – are mild steel. When you have an engine where the balance factor can at best only be a guess, it would be tiresome to have to strip the engine every time a different factor was tried.

At least one of the old factories did tests in a similar way, with tungsten plugs.

I arbitrarily increased the balance factor on my 650 twin of another popular make, by enlarging a hole at the big ends’ side of the flywheel with a drill poked in at the sump plate. It may have reduced vibration a bit.

G'day RD.Nice work mate.I agree balancing acts are time consuming. I'll have to consider that method next time the Cafe is down. I have to lower her bf as she walks backwards out the door when on the center stand when the throttle is blipped.Cheers

Started building in about 1977/8 a on average '52 A10 -built from bits 'n pieces never resto intended -maybe 'personalised'Have a '74 850T Moto Guzzi since '92-best thing I ever bought doesn't need a kickstart 'cos it bump starts sooooooooo(mostly) easyAustralia

mine dances backwards too it always has on the centre stand when I rev it , but it flys a lot better going forward when I resume to nutter on a beezer race mode forgot to say front wheel and forks bounce on tick over also. I have seen many that don't have tick over wheel bounce but maybe those that are without tick over bounce have vibes at revs where mine doesn't --- I don't know --not ridden another one.

It's all very well to talk about drilling holes in the flywheel but to do the job properly, i.e. dynamically, the crank has to be put onto a balancing machine.Metal is not just taken from the centre line of the flywheel but also from the bob weights.Static balancing is better than no balancing but dynamic is better again.Just my opinion for what it's worth.

Julian's service document seems to take very lightly, "Pull the engine. Drill some holes. Thoroughly clean out a bunch of swarf (not, "Swarfy") that's been distributed in, and stuck to the sides of, the oily crankcase. Put the engine back in. Test ride and repeat a couple of times." If you were to use the 'drill from below method' described maybe the whole thing is worth while. I could see it being easier to set up a bike inversion stand than going the engine removal route. All this said, I'll just be leaving the A7 to the machine shop for dynamic balancing (if I ever get around to it).

I'm still amazed how Julian comes through with the obscure documents.

Richard L.

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